1. Field of the Invention
Generally, the present invention relates to the formation of integrated circuits, and, more particularly, to the formation of metallization layers including highly conductive metals, such as copper, embedded into a dielectric material.
2. Description of the Related Art
In modern integrated circuits, minimum feature sizes, such as the channel length of field effect transistors, have reached the deep sub-micron range, thereby steadily increasing performance of these circuits in terms of speed and/or power consumption. As the size of the individual circuit elements is significantly reduced, thereby improving, for example, the switching speed of the transistor elements, the available floor space for interconnect lines electrically connecting the individual circuit elements is also decreased. Consequently, the dimensions of these interconnect lines have to be reduced to compensate for a reduced amount of available floor space and for an increased number of circuit elements provided per chip. In integrated circuits having minimum dimensions of approximately 100 nm and less, a limiting factor of device performance is the signal propagation delay caused by the switching speed of the transistor elements. As the channel length of these transistor elements is less than 100 nm, it turns out, however, that the signal propagation delay is no longer limited by the field effect transistors but is limited, owing to the increased circuit density, by the close proximity of the interconnect lines, since the line-to-line capacitance is increased in combination with a reduced conductivity of the lines due to their reduced cross-sectional area that is caused by the reduced available floor space. The parasitic RC time constants therefore require the introduction of a new type of material for forming the metallization layer.
Traditionally, metallization layers are formed by a dielectric layer stack including, for example, silicon dioxide and/or silicon nitride with aluminum as the typical metal. Since aluminum exhibits significant electromigration at higher current densities than may be necessary in integrated circuits having extremely scaled feature sizes, aluminum is being replaced by copper, which has a significantly lower electrical resistance and a higher resistivity against electromigration.
The introduction of copper, however, entails a plurality of issues to be dealt with. For example, copper may not be deposited in higher amounts in an efficient manner by well-established deposition methods, such as chemical and physical vapor deposition. Moreover, copper may not efficiently be patterned by well-established anisotropic etch processes and therefore the so-called damascene technique is employed in forming metallization layers including copper lines. Typically, in the damascene technique, the dielectric layer is deposited and then patterned with trenches and vias that are subsequently filled with copper by plating methods, such as electroplating or electroless plating.
A further issue with the copper technology is the ability of copper to readily diffuse in silicon dioxide. Therefore, copper diffusion may therefore negatively affect device performance, or may even lead to a complete failure of the device. It is therefore necessary to provide a diffusion barrier layer between the copper surfaces and the neighboring materials to substantially prevent copper from migrating to sensitive device regions. Thereby, the diffusion barrier layer may also serve to improve adhesion and impart enhanced mechanical stability to the structure.
Silicon nitride is known as an effective copper diffusion barrier, and is thus often used as a dielectric barrier material separating a copper surface from an interlayer dielectric, such as silicon dioxide. As previously noted, the device performance of extremely scaled integrated circuits is substantially limited by the parasitic capacitances of adjacent interconnect lines, which may be reduced by decreasing the resistivity thereof and by decreasing the capacitive coupling in that the overall dielectric constant of the dielectric layer is maintained as low as possible. Since silicon nitride has a relatively high dielectric constant k of approximately 7, compared to silicon dioxide (k≈4) or other silicon dioxide based low-k dielectric layers (k<4), frequently, barrier layers on the basis of silicon carbide are used. Moreover, silicon carbide may provide an enhanced interface bonding for low-k materials compared to silicon nitride.
Although copper exhibits superior characteristics with respect to resistance to electromigration compared to, for example, aluminum, the ongoing shrinkage of feature sizes, however, leads to a further reduction of the size of copper lines and thus to increased current densities in these lines, thereby causing a non-acceptable degree of electromigration despite the superior characteristics of copper. Electromigration is a diffusion phenomenon occurring under the influence of an electric field, which leads to a metal diffusion in the direction of the moving charge carriers, thereby finally producing voids in the metal lines that may cause device failure. In the case of copper, it has been confirmed that these voids typically originate at the copper/diffusion barrier interface and represent one of the most dominant diffusion paths in copper metallization structures. It is therefore of great importance to produce high quality interfaces between the copper and the diffusion barrier, such as the silicon nitride layer or the silicon carbide layer, to reduce the electromigration to an acceptable degree.
It turns out, however, that irrespective of the barrier material used, significant electromigration may be observed in modern integrated circuits, wherein this effect is further enhanced in the presence of elevated temperatures, mechanical stress, and the like, which represent typical operating conditions of modern integrated circuits. Thus, a further device scaling may result in a reduced device performance or in a premature device failure owing to increased metal diffusion along the interface between the barrier layer and the metal line.
In view of the problems with respect to electromigration of metals, such as copper, at interfaces to an overlying surface of a barrier layer, an improved technique is required that may eliminate or at least reduce some of the issues identified above.